The invention relates generally to the field of magnetic bubble technology (MBT), and more particularly to the structure and fabrication of bubble devices, especially those in which bubble logic is performed.
MBT involves the creation and manipulation of magnetic bubbles in specially prepared magnetic materials. The word "bubble" used throughout this text, is intended to encompass any single-walled magnetic domain, defined as a domain having an outer boundary which closes on itself. The application of a static uniform magnetic bias field orthogonal to a sheet of magnetic material having suitable uniaxial anisotropy causes the normally random serpentine pattern of magnetic domains to shrink into isolated, short cylindrical configurations or bubbles whose common polarity is opposite that of the bias field. The bubbles repell each other and can be moved or "propagated" by a magnetic field in the plane of the sheet.
Many schemes exist for propagating bubbles along predetermined channels at a precisely determined rate so that uniform data streams of bubbles are possible in which the presence or absence of a bubble at a particular position within the stream indicates a binary "1" or "0". MBT was originally envisioned in the form of a mass memory, but some of the most difficult problems have been encountered in the basic memory function of readout. It is possible, however, to minimize readout to a great extent by incorporating logic in the memory so that the magnetic bubbles representing information can be logically manipulated before readout is necessary to increase the quality or informational content of each bit of readout. The use of bubbles themselves as logic variables for performing logic operations is based on the fact that close magnetic bubbles tend to repell each other. Thus, if alternate paths with varying degrees of preference are built into a propagation system, the direction which a bubble on one channel ultimately takes may be influenced by the presence or absence of a corresponding bubble on another closely spaced channel.
Conventional bubble devices are prepared in composite monolithic structures referred to as "chips" in analogy to the structure of integrated electrical circuits. In field-accessed propagation systems, a thin overlay pattern of soft ferromagnetic elements is formed on a spacing layer over a sheet of magnetic bubble material. Nonmagnetic electrical conductors are also bonded to the spacing layer for use in bubble generation, transfer, readout and annihilation, for example. The thickness of the spacing layer has critical margins. It is well known that the spacing layer effects the degree of interaction or "coupling" between the bubbles and the overlay circuit. Magnetic bubbles tend to stay under the soft ferromagnetic elements of the circuit overlay because of the energy minimizing effect of containing the magnetic lines of flux emanating from the bubble domain, much like a keeper on the poles of a magnet. If the overlay material is too close to the bubble material, however, spurious bubble generation or "nucleation" can occur. If the spacing is too great, the interaction between the overlay and the bubbles may not be significant enough to be useful. The aim is thus to have the overlay close enough to the bubble material to guide the bubbles without adversely affecting them. There are optimal spacings for different kinds of circuit elements as well as for different materials, thickness, widths and other parameters.
Generally speaking, the degree of coupling in a circuit designed only for storage or memory should be high since the basic characteristic of field-accessed bubble memory is orderly propagation, and a high magnetic interaction between the overlay and the bubbles serves to keep the bubble from straying from the propagating track. On the other hand, in circuits designed to perform logic by means of bubble-to-bubble repulsion, there are occasions when it is desirable to deflect one bubble onto an adjacent track, and in this case the deflected bubble should be less strongly attracted or coupled to its original circuit element while the deflecting bubble should be strongly coupled to its circuit element.